1. Field of the Invention
The present invention relates to a diagnostic system for an exhaust gas recirculation device.
2. Description of the Related Art
Conventionally, to reduce the amount of NO.sub.X in the exhaust gas, an exhaust gas recirculation (hereinafter referred to as an EGR) device, in which the exhaust gas is recirculated into the intake passage of an engine via the EGR passage, is employed. In such an EGR device, an EGR control valve is normally arranged in the EGR passage to control the amount of EGR gas to be fed into the intake passage. However, in such an EGR device, the EGR control valve can be damaged or become clogged, and thus the supply of the EGR gas is stopped, and if the supply of the EGR gas is not restored, a large amount of NO.sub.X will be continuously discharged. In addition, normally, a driver will not notice the stoppage of the supply of EGR gas.
Consequently, to diagnose such a malfunction of the EGR device, a diagnostic system is known in which an exhaust gas temperature sensor is arranged in the EGR passage downstream of the EGR control valve (Japanese Unexamined Utility Model publication No. 49-64623 or No. 50-67220). In this diagnostic system, if the temperature in the EGR passage downstream of the EGR control valve does not increase beyond a fixed temperature, under an engine operating state wherein the recirculating operation of the EGR gas is to be effected, it is determined that a malfunction of the EGR device has occurred. This diagnostic method makes use of the phenomenon that, when the recirculating operation of the EGR gas is carried out, the temperature in the EGR passage will increase beyond the fixed temperature.
However, in this diagnostic system, where the temperature of the exhaust gas itself is low, even if control of the supply of the EGR gas is correctly carried out, the temperature in the EGR passage will not increase beyond the fixed temperature. As a result, it is determined that the EGR device has malfunctioned, and thus the diagnostic system will make an incorrect diagnosis. To avoid this problem, the fixed temperature used to determine the occurrence of a malfunction can be lowered, but in this case it is always determined that the operation of the EGR device is normal, and thus it is impossible carry out a diagnosis of the EGR device.